In recent years, ongoing increase to capacity and speed is required in response to implementation of broadband to mobile communication services. In this regard, a 3rd-generation mobile communication system represented by W-CDMA (Wideband Code Division Multiple Access) or the next-generation mobile communication system, which is an alternative to a 3.5th-generation mobile communication system, has been commercialized all over the world. Also in Japan, frequency allocating for a 3.9th-generation mobile communication system, which is positioned as a mobile communication system leading to a 4th-generation mobile communication system, has begun. In the 3.9th-generation mobile communication system, LTE (Long Term Evolution) is the most powerful candidate for a standard led to the 4th-generation mobile communication system.
In the LTE, OFDMA (Orthogonal Frequency Division Multiplexing Access) is adopted for downlink communication toward a radio terminal from a radio base station, and SC-FDMA (Single Carrier Frequency Division Multiple Access) is adopted for uplink communication toward the radio base station from the radio terminal. In these multiplexing methods, user multiplexing is achieved by arranging a radio resource in the two-dimensions of frequency and time.
A frequency band, which is a downlink radio resource, is divided in units of a resource block (RB). The RB includes a control information channel (PDCCH: Physical Downlink Control CHannel), which is a time slot as a radio channel for downlink control information transmission, and a shared data channel (PDSCH: Physical Downlink Shared CHannel) which is a time slot as a radio channel for downlink user data transmission.
In the LTE, when a radio base station allocates CCE (Control Channel Element: a radio resource for a radio terminal) to each radio terminal, the radio base station sets an aggregation level (AL), which indicates the number of continuous CCEs to be ensured, with respect to the PDCCH for control information transmission. Thus, it is possible to perform allocating control corresponding to a radio state between the radio terminal and the radio base station.
In this case, in a CCE allocating position for transmitting control information toward the radio terminal, a certain level of allocating position is limited in advance as a search space for each radio terminal.
The search space is set to be different between radio terminals to the extent possible based on RNTI (Radio Network Temporary Id), which indicates unique information of a radio terminal, the number of subframes, and AL corresponding to the radio terminal (for example, see Non-Patent Documents 1 and 2).